1. Field of the Invention
The present invention relates to a photovoltaic device, and more particularly to a surface coating which has excellent weather and impact resistance and which is applied to a photovoltaic device having a semiconductor layer and an electrode layer serving as photoelectric conversion members formed on a base member thereof, and to a photovoltaic device, the output characteristics of which are improved because light to be absorbed by the semiconductor layer can be utilized effectively by scattering of the incident light.
2. Related Background Art
Recently, it has been predicted that the earth will be warmed by a greenhouse effect due to an increase in CO.sub.2 and therefore there arises a desire for a clean energy source. Since nuclear power generation has encountered an unsolved problem in disposing of the radioactive waste, greater safety and another source of clean energy have been required. Among the various clean energy sources expected for future use, solar cells have been developed due to cleanness, safety and handling facility.
Of the various materials for solar cells, amorphous silicon and copper indium selenide have been energetically studied because a large cell can be manufactured and the manufacturing cost can be reduced.
The solar cells sometimes use substrates made of metal such as stainless steel because of its capability of reducing weight, excellent weather and impact resistance, and flexibility. If a metal substrate made of, for example, stainless steel is employed, at least the light incident surface must be covered with a coating, which is transparent with respect to visible light, in a manner different from the case in which a glass substrate is employed. As shown in FIG. 13 in a schematic cross-sectional view which illustrates a conventional solar cell module, the conventional solar cell module comprises a solar cell 1300 laminated with fluororesin films 1302 exhibiting excellent weather resistance while interposing adhesive layers 1301 also serving as fillers and made of EVA (ethylene vinyl acetate copolymer).
However, the fact that the EVA and fluororesin film have low hardness causes scratching damage and the like to be easily generated. Accordingly, the EVA layers and the fluororesin films are thickened in order to protect the body of the solar cell from damage. In order to improve the hardness and strength, a structure is sometimes employed in which unwoven glass fabric such as crane glass is used as a filler embedded in the EVA layer. "Crane glass" is a nonwoven fiberglass web produced by Crane and Co., Inc., which is available as a fiberglass sheet 0,127 mm in thickness. However, the weight cannot be reduced if the EVA serving as the adhesive agent and as the filler or the total thickness of the EVA layer and the crane glass is too thick.
In an outdoor experiment to which the resin film laminated-type solar cell module is subjected, small spaces are undesirably formed between the resin film and the adhesive agent for adhering the resin film to the solar cell and therefore the resin film is locally separated.
In order to overcome the foregoing problem, a method has been disclosed in Japanese Patent Laid-Open Application No. 59-73942. Described therein is a process wherein a surface which is in contact with the adhesive layer made of a fluororesin film or an ethylene chloride trifluoride resin film in subjected to RF sputtering etching. Furthermore, the solar cell is covered with the foregoing films and the adhesive agent to improve the adhesive strength. However, the structure is inevitably multi-layer and an RF sputtering etching process must be performed. Therefore, a complicated manufacturing process is required.
Another method for further reducing the weight has been attempted by directly covering the light incident surface of the solar cell with a fluororesin paint exhibiting weather resistance in place of the fluororesin film and the adhesive layer. However, the fact that the fluororesin has insufficient hardness causes a problem of damage. Therefore, the aforesaid method cannot be adapted to a solar cell used outdoors.
A method of improving the photoelectric conversion efficiency of a photovoltaic device such as a solar cell incorporating a semiconductor layer for generating photovoltaic force and electrodes has been disclosed. The method utilizes an arrangement wherein light incident on the photovoltaic device is scattered to lengthen the optical path in the semiconductor layer, to increase the amount of light absorbed in the semiconductor layer, and to increase the short-circuit current.
The structure of the photovoltaic device for scattering light incident on the photovoltaic device is exemplified by: (1) a transparent electrode having projections and pits formed on a transparent substrate such as a glass substrate; (2) a light reflection layer having projections and pits formed on a substrate; and (3) a semiconductor substrate having projections and pits formed therein. The structures using the glass substrate or the semiconductor substrate scatter light on the surface of the light incident side thereof. The structure having projections and pits formed in the light reflection layer scatters the light on the side opposite the light incident side thereof.
The structure incorporating the glass substrate to scatter the light on the surface of the light incident side has an arrangement such that light is incident on the semiconductor layer through the substrate. Therefore, a transparent electrode having projections and pits is formed with heat at a temperature of about 300.degree. C. or more prior to forming the semiconductor layer.
On the other hand, the photovoltaic device having the structure in which light is incident on the semiconductor layer through a portion opposing the substrate is manufactured as follows: first, a thin semiconductor layer is formed on the substrate; and then, the light scattering structure is formed. Therefore, the temperature of the substrate cannot be raised to a level higher than the temperature at which the semiconductor layer is formed, during the forming of the transparent electrode. What is worse, the transparent electrode having projections and pits cannot easily be formed though they can easily be formed in the glass substrate. If the transparent electrode also serves as an anti-reflection layer, the reflectance is usually lowered over a wide wavelength range by thinning the optical thickness (nd) of the transparent electrode to .lambda./4 (.lambda. is a wavelength which minimizes the reflectance). In the foregoing case, the transparent electrode is too thin to form projections and pits therein.
If the semiconductor layer having projections and pits for scattering light formed thereon is used, the substrate must be handled delicately. Therefore, the number of manufacturing steps increases and the manufacturing cost is increased accordingly.
As described above, the photovoltaic devices, except for those having a structure in which light is incident on the semiconductor layer from the glass substrate, encounter a problem in that the structure for scattering light on the surface of the light incident side cannot easily be employed due to limitations needed to maintain manufacturing efficiency and to reduce cost.
However, there has been a desire to improve the photoelectric conversion efficiency of photovoltaic devices which use a low-cost substrate such as a stainless steel substrate or a synthetic resin film, except for glass substrates, in order to practically manufacture a low-cost photovoltaic device which can be adapted to wide use typified by electric power generation. Therefore, there has been a desire to form the structure for scattering light on the surface of the light incident side of the photovoltaic device using a low-cost substrate beside glass substrates.